docs: def API520_SH(T1, P1):
      r'''Calculates correction due to steam superheat for steam flow for use in
      API 520 relief valve sizing. 2D interpolation among a table with 28
      pressures and 10 temperatures is performed.
  
  
      Parameters
      ----------
      T1 : float
          Temperature of the fluid entering the valve [K]
      P1 : float
          Upstream relieving pressure; the set pressure plus the allowable
          overpressure, plus atmospheric pressure, [Pa]
  
      Returns
      -------
      KSH : float
          Correction due to steam superheat [-]
  
      Notes
      -----
      For P above 20679 kPag, use the critical flow model.
      Superheat cannot be above 649 degrees Celsius.
      If T1 is above 149 degrees Celsius, returns 1.
  
      Examples
      --------
      Custom example from table 9:
  
      >>> API520_SH(593+273.15, 1066.325E3)
      0.7201800000000002
  
      References
      ----------
      .. [1] API Standard 520, Part 1 - Sizing and Selection.
      '''
      if P1 > 20780325.0: # 20679E3+atm
          raise Exception('For P above 20679 kPag, use the critical flow model')
      if T1 > 922.15:
          raise Exception('Superheat cannot be above 649 degrees Celcius')
      if T1 < 422.15:
          return 1. # No superheat under 15 psig
      return float(bisplev(T1, P1, API520_KSH_tck))